The present invention relates to a method for producing polysilicon thin film transistors (TFT) which exhibit low reverse leakage under reverse bias conditions. The instant invention further is concerned with the fabrication of low leakage, high performance polysilicon thin film transistors in which fabrication temperatures that are employed are sufficiently low so as to allow the use of commercially available glass substrates.
As described in Ovshinsky et al, U.S. Pat. No. 4,843,443 thin film field effect transistors, are found to be particularly useful in electronic matrix arrays such as employed in liquid crystal displays and high density memories. When such transistors are employed in the liquid crystal displays, particularly for projection television systems, it is important that leakage current of these transistors be held to the minimum. It should be noted from U.S. Pat. No. 4,904,056 that leaking current causes the voltage stored in the liquid crystal capacitors employed in the liquid crystal matrix to change as a result of which display performance is degraded.
Tuan, U.S. Pat. No. 4,752,814, shows the use of a high voltage amorphous silicon thin film transistor, a gate and source electrode laterally displaced from a drain electrode for the use in amorphous silicon transistor switches employed for driving electrographic stylii. Here too, it is necessary that the reverse current flow of the transistor be minimized in order to prevent a breakdown of the transistor and an interrupt functioning of the stylii.
Fabrication of polysilicon thin film transistors generally requires the use of temperatures of as high as 1000.degree. C. As a result, the use of expensive quartz substrates are required.
A thin film transistor fabricated at temperatures below 650.degree. C., which allows the use of glass substrates, has been reported by H. Ohshima and S. Morozumi (IEDM proceedings, Washington, 1989 page 157). It has been found the characteristics of these devices are quite inferior when compared to the thin film transistors produced at high temperatures. In addition, these low temperature fabricated transistors found to exhibit relatively high leakage current.
Troxell et al, U.S. Pat. No. 4,851,363 shows a polysilicon thin film transistor as an alkaline earth alumino-silicate glass having a annealing point of approximately 800.degree. C. Moreover the transistors made by the method of this patent do not exhibit any reduction in leakage current. Additionally during one stage of the method a temperature of at least 800.degree. C. is employed, a temperature too high to allow use of many commercially available glasses.
Malhi et al, European Patent Specification 0129037 shows a method of reducing leakage current in thin film transistors by use of a hydrogen plasma treatment. While their technique reduces the leakage current flow a more significant decreased is desired, particularly for devices fabricated at low temperatures since low temperature fabrication leads to sharp (not well diffuse) junctions leading to higher electric fields and more leakage.